There are almost 400 billion trees in the Amazon River basin, “close to the number of stars in the Milky Way galaxy,” says Nigel Pitman, a Field Museum visiting scientist who is second author of a new study that provides the best answer yet to this difficult-to-quantify question.

The study, published today in the journal Science, calculates that there are roughly 16,000 tree species in the vast and varied region, roughly the size of the continental U.S., but that just 227 of those species, including Brazil nut, chocolate, rubber, and acai berry trees, comprise about half of all the trees.

Before this, the Amazon was “a system that we couldn’t answer even really simple questions about,” says Pitman, the Robert O. Bass Visiting Scientist at the Field. “We knew there were lots of species down there. We couldn’t tell you how many, which were common and which were rare across the basin, where they were common, where they were rare.”

Such knowledge is important to better understanding the region that plays such a key role in the global climate and will help make the study of tree life less daunting, says Pitman, who will help the museum get a Science Newsflash temporary exhibit explaining the study ready for Monday.

Confronting 16,000 species as a scientist “is enough to cause an existential crisis that sort of makes you throw up your hands,” says Pitman. “But going down to the same forest and finding out it’s (mainly) 200 species, or in some regions 70, that has the opposite effect. That really allows you to focus and say, ‘We can do a whole lot.’”

To build their estimate, Pitman, lead author Hans ter Steege of the Naturalis Biodiversity Center in the Netherlands and their colleagues combined data from almost 1,200 tree population surveys taken in small areas, typically 100 meters square, over the last decade by scores of individuals and institutions. These surveys include some of the work done by the museum’s “rapid inventory” conservation team.